X-ray photoemission study of CoFeB∕MgO thin film bilayers

Abstract

We present results from an X-ray photoemission spectroscopy (XPS) study of CoFeB/MgO bilayers where we observe process-dependent formation of B, Fe, and Co oxides at the CoFeB/MgO interface due to oxidation of CoFeB during MgO deposition. Vacuum annealing reduces the Co and Fe oxides but further incorporates B into the MgO forming a composite MgB x O y layer. Inserting an Mg layer between CoFeB and MgO introduces an oxygen sink, providing increased control over B content in the barrier.

Depth analysis of boron diffusion in MgO/CoFeB bilayer by x-ray photoelectron spectroscopy

“…4 as a function of sput- Further analyzing the peak position in Fig. 3͑a͒, we can find that the position of B oxide between 22 and 24 min sputtering time is almost constant and shifted by about 1 eV to higher BE compared with the B oxide position in the reference sample, which agrees with the results of Read et al 7 The higher BE means that the B in MgO is in a higher oxidation state, closer to B 3+ , than B of the boron oxide in the oxidized CoFeB layer ͑BO x ͒. This B 1s position in MgO also agrees with the value measured by Ong et al 19 on B 2 O 3 ͑Ϫ193.6 eV with respect to the Fermi level͒.…”

“…It leads to two possibilities: ͑i͒ B could be in a substitutional position, occupying the Mg vacancies to form a ͑MgO͒ x ͑B 2 O 3 ͒ y compound 11 or ͑ii͒ it could be in the amorphous form of B 2 O 3 localized in the MgO grain boundaries. The large concentration of B distributed in our MgO layer after annealing agrees both with the claims of Read et al 7 that an intermediate oxide Mg x B y O could form after the annealing procedure and with the recent study of boron distribution by Cha et al by using EELS. 11 However, to end up this discussion, further experiments are required.…”

“…In fact, Read et al 7 and Cha et al 8 have found that B diffusion is very dependent on the growth method. B diffusion is more pronounced in rf-sputtered MgO layers than for e-beam evaporated MgO.…”

“…[6][7][8][9][10][11][12][13][14][15][16] However, different conclusions came out from the experiments. Some groups reported that a considerable quantity of B diffuse into the MgO barrier after annealing, [6][7][8][9][10][11] while some others observed that boron tends to segregate at the MgO/CoFeB interface and form BO x after the crystallization of CoFeB layer. [12][13][14][15] Moreover, recently, the group of Miyajima et al 16 found that the crystallization of CoFeB layers was strongly dependent on the capping materials.…”

Interlayer exchange coupling and current induced magnetization switching in magnetic tunnel junctions with MgO wedge barrier

“…In our experiment, during an annealing process the B atoms from CoFeB electrode can diffuse into the MgO barrier 15 ͑which is not the case in pure Fe electrodes used in experiment in Refs. 5 and 6͒, which can result in other exchange mechanisms.…”